A short circuit in low, medium and high voltage applications, such as switchgears and industrial electrical installations, is typically accompanied by the occurrence of an electric arc. An arc occurrence always leads to considerable damages to equipment and if people are present at the time of arcing they run the risk of getting severe or lethal injuries, unless the arc is extinguished very fast.
It is known to arrange supervision and safety equipment in electrical installations in the form of an arc monitor. The object of the arc monitor is to prevent or limit the damages to equipment and people in the event of an arc occurrence, by tripping the circuit breaker of the switchgear within a couple of milliseconds from the formation of the arc. In order to identify the occurrence of an arc the arc monitor comprises a number of arc detectors, which are to be located at suitable locations within the electrical installation wherever arcing is likely to occur.
A short circuit may for instance happen as a consequence of a mistake or an accident when an operator drops a tool on, or forgets a tool abutting, live parts of the switchgear, or as a consequence of bad electric connection which may lead to heat and spark formation, or as a consequence of animals finding their way into the switchgear and causing a short circuit by contacting live parts.
Upon such cases, when an arc occur, the arc monitor is arranged to quickly trip e.g. the main circuit breaker of the switchgear, in order to shorten the burning time of the arc. Thereby the risk of damages to equipment and people is prevented or limited, and the accessibility to the switchgear for the maintenance personnel is increased, such that the break down may be taken care of as quick as possible. Thus, the short circuiting tool or animal may be removed, or the loose screw may be fastened.
A conventional arc monitor comprises a fixed number of arc detector input terminals, e.g. ten, and may thus only be equipped with the same number of arc detectors. If the electrical installation, in which the supervision and safety equipment is arranged, is of considerable size more than ten arc detectors may be needed. This means that several arc monitors must be used to cover the entire electrical installation, which arc monitors are connected to the circuit breaker in parallel with each other. The use of several arc monitors is space requiring and the total cost is unnecessary high. In addition, a conventional arc monitor is only operatively connected to a single circuit breaker, usually the main circuit breaker of the electrical installation, e.g. of the switchgear. In electrical installations of considerable size, e.g. an installation having a number of separate compartments having separate or shared sub circuit breakers, it is sometimes unneeded to shut down the entire electrical installation in the event of an arc occurrence in one of the compartments. It is a problem that operational disturbances will unnecessarily affect customers/users if the entire electrical installation is shut down. In addition, after a total shut down, an electrical installation of considerable size may need days before full electrical load may be taken out from the electrical installation, i.e. the electrical load must be ramped up, which will further affect the customers/users that were already from the beginning unnecessarily affected. If a separate or shared sub circuit breaker of a compartment of an electrical installation is required to be separately controlled, conventionally a separate arc monitor must be dedicated that task, even though only a few arc detectors of that arc monitor is needed and/or used to supervise said compartment. This entails that much of the equipment is unutilized.
The present invention aims at obviating the aforementioned disadvantages of previously known arc monitors, and at providing an improved arc monitor. A primary object of the present invention is to provide an improved arc monitor of the initially defined type that may be adapted for the needs of the specific electrical installation.
It is another object of the present invention to provide an arc monitor, which is less space requiring in large electrical installations.
It is another object of the present invention to provide an arc monitor, which is less expensive than conventional arc monitor solutions when used in large electrical installations. It is yet another object of the present invention to provide an arc monitor, to which different types of arc detectors may be connected. It is also an object of the present invention to provide an arc monitor, the function and status of which may be tested and checked remotely.